Internal combustion engines are subject to strict emission limits. Approaches to reducing emissions include improved combustion designs and fuel modifications, but these improvements have fallen short of meeting emissions limits. Other approaches involve the use of exhaust aftertreatment devices, which have achieved significant emissions reductions.
For diesel engines, which are conventionally run at a lean air-fuel ratio, the main pollutants of concern are oxides of nitrogen (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by sulfur in fuel, and components of unburned fuel and oil.